It shouldn't be this hard. Or maybe it should, to keep us paid so well.
You have to enable the secret handshake
The menu like two gear wheels for the tricky techie stuff is disabled by default.
Press the menu button.
Down on the D-keys to the spanner icon
Right, then
Down 4 times to two gearwheels menu display
Right once it'll say "OFF"
Down once to turn it to "ON"
Press OK button
Now the menu, press left and up
In the gearwheel menu click right to "A"
Then down to "B" Button/dial
(nearly there!)
Right
Down twice to AEL/AFL
right to mode
in S-AF mode click right
scroll through to mode 3
AEL now does single AF
Shutter half press AEL
press OK
Finally, if you want, the Fn button for autofocus.
Down 6 times
AEL ->
AFL <- Fn
should be OFF by default.
Click right and set it to ON.
press OK.
Now finally you have S-AF on the Fn button.
Press Menu twice to get out of the menu.
Whew !!
I think most people are not going to find it if the gearwheel menu is turned OFF by default.